Method for guarding electrical regions having potential gradients

ABSTRACT

A multi-potential guarding technique for preventing electrical charge exchange to or from an electrical system containing potential gradients and sources of electrical noise. An inner electrically conductive guard shield surrounds the system to be guarded. The electrical potential of the inner electrically conductive guard shield is such that there is ideally no net current flow to the guard shield from the electrical system therewithin. An outer electrically conductive guard shield surrounds the inner electrically conductive guard shield. An operational amplifier drives the outer electrically conductive guard shield to an electrical potential that is substantially equal to that of the inner electrically conductive guard shield so that there is substantially no potential difference between the inner and outer guard shields which has the effect of producing a high insulation impedance around the guarded system. The multi-potential guarding technique of this invention has application in a gradient-guard configuration and a charge-channel configuration.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a method for preventing electrical chargeexchange within an electrical system containing potential gradients(i.e. the voltage is not constant). Accordingly, leakage and noisecurrents are prevented from escaping or entering a conductor when theinsulation thereof might be inadequate (e.g. particularly in wetenvironments or at high temperatures). A principal application of thepresent invention is to prevent leakage from or into a conductor thatsupplies current to or voltage from a remote sensor (e.g. a resistancetemperature detector) where the calibration of the sensor depends uponcurrent through or voltage across the sensor.

2. Background Art

Guarding is a well known technique for reducing the effects ofelectrical current leakage through electrical conductors as aconsequence of imperfect insulation and shunt capacitance in low leveland highly precise measurements and reducing common mode interference.These effects result from the finite impedance to ground and to nearbyconductors that accumulate throughout a system.

Guarding has been traditionally applied to protect regions containing asingle electrical potential. Driven guards typically take the form of anelectrostatic enclosure and are often referred to as shields or guardshields. Any electrical circuit will necessarily develop voltagegradients when it is placed in operation. A non-self-generating voltagegradient example is a current carrying conductor that develops apotential gradient along its length. Some self-generating voltagegradient examples include parasitic potentials that develop within aguarded region via tribo and radiation-electric effects as well as thechanging potential on mechanically variable cable capacitance holding anessentially constant charge. By way of further example, when usingplatinum resistance temperature detectors to obtain accuratemeasurements in high temperature environments, it was found that sometemperatures were so high that available insulation compounds developedunacceptable levels of leakage. Such leakage adversely effected theaccuracy of measurement and increased uncertainties relating thereto.

Accordingly, what is needed is a more effective and reliable guardingtechnique whereby almost any circuit or conductor can experienceessentially no leakage in operation, even where poor insulation existsbetween the circuit and its environment and where parasitic electricalcharges exist between a conductor and its environment. Reference may bemade to the following U.S. patents which describe methods for guardingelectrical regions:

    ______________________________________                                        3,866,093           Kusters et al.                                            4,115,790           Tsunefuji                                                 5,434,512           Schwindt et al.                                           5,457,398           Schwindt et al.                                           ______________________________________                                    

SUMMARY OF THE INVENTION

While conventional guarding techniques prevent regions containing asingle electrical potential from exchanging electrical charge with theremaining environment as a consequence of imperfect insulation, thepresently disclosed invention relates to an improved multi-potentialguarding technique that prevents electrical charge exchange from regionscontaining potential gradients (i.e. the voltage is not constant). Themulti-potential guarding technique described herein is adapted tosuppress noise originating within a cable that connects a transducer tosignal conditioning electronics as well as noise originating between anactive circuit and its associated guarding conductors. Gradient guardand charge channel versions of the improved guarding technique aredescribed.

Experimental results demonstrate that a current carrying conductor,having a significant IR drop and protected within a charge channel, canhave the same entry and exit current despite poor and varying insulationperformance from the conductor to the environment. Additionalexperimental results demonstrate that the improved guarding technique ofthis invention prevents potentials generated between a conductor and itsguard from influencing external signals. Moreover, it will be shown thatthe improved guarding technique can significantly reduce the externaleffects of noise potentials within the guarded region which may arisefrom tribo-electric effects, radiation exposure, electromechanicalinputs, and other noise sources.

In general terms, a method is disclosed for guarding an electricalsystem that is located within an inner electrically conductive guardthat has an electrical potential. Ideally, the electrical potential ofthe inner guard is such that there is no net current flow to the innerguard from the system located therewithin. The system protected by theinner electrically conductive guard typically contains potentialgradient and noise sources. The inner electrically conductive guard issurrounded by an outer electrically conductive guard. The outer guard isdriven by means of an operational amplifier to an electrical potentialthat is substantially identical to the electrical potential of the innerelectrically conductive guard, such that there is substantially nopotential difference between the inner and outer guards. A firstelectrical impedance exists between the inner and outer electricallyconductive guards, and a second electrical impedance exists between theouter electrically conductive guard and the environment (i.e. electricalground).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic circuit illustrating the conventional guardingtechnique for reducing the effects of electrical current leakage from aprotected region containing a single electrical potential;

FIG. 2 shows a schematic circuit to illustrate a multi-potentialguarding technique according to the present invention to cancel leakageand certain internal voltage effects by means of inner and outer guardshields;

FIG. 3 shows a schematic circuit to illustrate a charge-channelapplication of the multi-potential guard technique illustrated in FIG.2;

FIG. 4 shows a schematic test circuit to demonstrate the charge channelapplication of the multi-potential guarding technique of FIG. 3; and

FIG. 5 shows a schematic test circuit to demonstrate an aspect of themulti-potential guarding technique of the present invention.

DETAILED DESCRIPTION

The conventional or single potential guarding technique cancels leakageeffects by requiring conductors to be at the same potential. Under theseconditions there will be no current between the conductors regardless ofthe impedance between them. FIG. 1 of the drawings illustrates the knownsingle-potential driven guard shield 10 applied to a conductor 1 whichsenses a signal from a high-impedance source 2.

In FIG. 1, an operational amplifier 4 is connected as a unity-gainbuffer which causes the guard shield 10 to assume the same potential, e,as the signal conductor 1. Because there is no potential differencebetween the signal conductor 1 and the guard shield 10, no current willflow through insulation impedance Z₁ regardless of the potential ofsource 2. Current that could have leaked from the conductor 1 to groundis supplied instead by the output of the operational amplifier 4 throughZ₂, thereby preserving all of the signal available from source 2. Thisapproach is widely known as the driven-guard technique to compensate forelectrical current leakage through the insulation impedance Z₁ of theconductor 1.

According to one aspect of the present invention, a multi-potentialguarding technique cancels leakage and certain internal voltage effectsby means of inner and outer guard shields 12 and 14 as illustrated inFIG. 2 of the drawings. Desired electrical potentials e₁ and e₂ as wellas noise potential, e_(n), exist within the inner guard shield 12.Without an effective guard, these potentials would cause current to flowto ground through any distributed impedance, especially leakage throughimperfect insulation. For purpose of illustration, the general case ofcircuit impedances and leakage paths are represented in lumped form inFIG. 2 by various impedances Z_(A) through Z_(E) within the inner guardshield 12. Impedance Z₁ exists between the inner and outer guard shields12 and 14 and impedance Z₂ exists from the outer guard shield 14 to theenvironment.

In one case, the inner guard shield 12 is caused to assume a "balance"potential such that all leakage currents flowing from the more positivepotentials within the circuitry to the inner guard shield 12 are matchedby other currents from the inner guard shield 12 back to the morenegative potentials in the circuitry. In other words, the potential ofthe inner guard shield 12 is such that the net current flow to the innerguard shield 12 from the circuitry within shield 12 is substantiallyzero. The balance potential will be somewhere between the extremes ofpotential within guard shield 12. The outer guard shield 14 is thenarranged to assume substantially the same potential as that of the innerguard shield 12.

This situation is similar to the concept of the conventional drivenguard illustrated in FIG. 1. However, the signal conductor 1 of FIG. 1now becomes the shell that contains the volume enclosed by the innerguard shield 12 of FIG. 2. Conventional guard theory applies to thecircuit node 13 consisting of the single-potential shell of this innerguard shield 12. Because there is substantially no potential differencebetween the inner and outer guard shields 12 and 14, substantially nocurrent will flow through impedance Z₁, thereby preserving whateversignal potential exists on the inner guard shield 12.

If the above conditions are met, under ideal conditions, there will beno net current flow from the potentials within the inner guard shield 12to the inner guard shield 12 and no current between the inner and outerguards regardless of the potentials within the inner guard shield 12.Under these same ideal conditions, no current will flow from thepotentials within the inner guard shield 12 to the outer guard shield 14or in other conductors or to ground regardless of the finite insulationimpedances distributed within the inner guard, as well as impedance Z₁and Z₂.

In practice when the outer guard shield 14 is driven to a potentialwhich is substantially equal to that of the inner guard shield 12,essentially no current can flow between the two guards. The inner guardshield 12 can be analyzed as a circuit node 13 into which the sum of thecurrents must be zero. So, if no current can leave the inner guardshield through impedance Z₁, then the inner guard circuit node 13 mustassume a potential such that the currents entering this node from thevarious potentials within the inner guard shield 12 sum to zero. In thisregard, it may be appreciated that driving the outer guard shield 12from the potential of the inner guard shield 14 causes the inner guardto automatically assume the balance potential.

The inner and outer guard shield arrangement illustrated in FIG. 2 toguard a region containing a potential gradient is similar to thebox-within-a-box construction commonly used in sensitive instruments.However, in accordance with this invention, a unity gain operationalbuffer amplifier 16 having essentially infinite input impedance (e.g.10⁶ ohms) drives an outer electrically conductive (e.g. copper oraluminum) sheet metal box (i.e. outer guard shield 14) with thepotential assumed by an inner box (i.e. inner guard shield 12).Operational amplifier 16 is located outside the outer guard shield 14.The inner electrically conductive sheet metal box is surrounded by andspaced from the outer box. The inner guard node 13 is connected througha small hole in the outer guard shield 14 to the positive non-invertinginput to operational amplifier 16, while the negative inverting input ofoperational amplifier 16 is connected in a feedback loop with the outputthereof and outer guard node 15. Extremely good insulation is ordinarilyused between the inner and outer conductive boxes of the instrument.But, with the multi-potential guard technique of FIG. 2, even poorinsulation between the inner and outer shields 12 and 14 will appear tohave essentially infinite impedance. Thus, currents that would typicallyflow from the region within the inner guard shield 12 to the environmentare now automatically provided by the output of operational amplifier16.

Two general classes of applications for the multi-potential guardtechnique described above are gradient guards and charge channels.Gradient guards prevent current from flowing between the environment andthe guarded circuitry. Charge channels cause the current injected into aguarded region to be channeled such that all of the injected currentexits in the region at only the desired point or points.

The gradient guard application of the multi-potential guard technique isillustrated in FIG. 2. As long as a noise source 18 does not alter theother potential gradients within the inner guard shield 12, the effectsof noise will be eliminated and no charge will be exchanged between theinner guard region and its external environment. This feature can reducenoise experienced within a signal carrying conductor 1 when variousnoise-generating effects cause the insulation to accumulate a charge.

The gradient guard application of FIG. 2 applies to the two dimensionalsurface-guard case as well as to the three dimensional volume-guardcase. Therefore, the surface of a printed circuit board or integratedcircuit can benefit from a gradient guard as well as circuitry containedwithin a guarded volume.

In accordance with a two dimensional surface construction, a printedcircuit is laid out on the usual non-conductive printed circuit board.The printed circuit is surrounded by an inner electrically conductivetrace (i.e. inner guard shield 12), and the inner electricallyconductive trace is surrounded by and spaced from an outer electricallyconductive trace (i.e. outer guard shield 14). The double guard shieldsformed by the inner and outer traces are driven by an operationalamplifier that is connected in the same manner as the amplifier 16 ofFIG. 2.

In accordance a three dimensional volume construction, a wire thatcarries information is surrounded along its entire length by an innerelectrically conductive braided or foil covering (i.e. the inner guardshield 12), and the inner covering is surrounded by and spaced from anouter electrically conductive braided or foil covering (i.e. the outerguard shield 14). The double shielded cable formed by the inner andouter covering is driven by an operation amplifier that is connected inthe same manner as the amplifier 16 of FIG. 2.

A gradient guard can be applied to equipment located in poorly-insulatedand charge-developing environments. Examples are areas with highhumidity and/or temperature and contamination or ionization as well asareas or volumes that include parasitic energy sources. There can evenbe a conductive liquid between the inner and outer guards and betweenthe outer guard and the environment. The operational amplifier 16driving the outer guard shield 14 supplies the necessary current to theenvironment around the outer guard as long as the conductivity of theliquid is significantly less than the conductivity of the guard shields12 and 14.

The charge-channel application of the multi-potential guard technique isillustrated in FIG. 3. This application also applies to thetwo-dimensional surface-guard case as well as to the three-dimensionalvolume-guard case. Distributed wire and insulation resistances 20 and 22are represented in lumped-parameter form. The double guard inner andouter shields 24 and 26 are driven by an operational amplifier 28connected in the same manner as the amplifier 16 of FIG. 2. Noisevoltages (not shown) can also exist within the inner guard shield 24.Ideally, when no current can flow through impedance Z₁ then, aspreviously disclosed when referring to FIG. 2, the inner-guard circuitnode 25 must assume the potential that results in no net current flowfrom the current-carrying conductor 30. Therefore, input current i₁ mustequal exit current i₂. A perfectly insulated conducting region leaks nocharge to its surrounding environment and will deliver all mobilecharges that are injected at one point (and not stored on the conductor)to one or more other points where they will exit from the conductor 30.A real conductor in a charge channel appears to be embedded inessentially perfect insulation, even if noise sources exist within theinner guard.

One particular application for the charge channel technique of FIG. 3 isexcitation wiring through hostile environments to platinum resistancethermometers (PRTs). The charge channel technique can prevent insulationleakage from allowing current to shunt around the sensing element (e.g.such as the electrical load impedance designated 42 in FIG. 4) andthereby cause its apparent resistance (and indicated temperature) to belower than its actual resistance. In fact, it is necessary to employ acharge-channel guard around only one conductor carrying current in anAnderson loop signal conditioner.

Four significant limitations of the multi-potential guarding techniquehave been identified. First, while this technique will theoreticallyeliminate charge transfer from the inner guard to the environment, itcannot guarantee that potential gradients within the inner guard areunaffected by all internal impedance variations or noise sources.

Second, for best performance, each conductor in a signal pair or anexcitation pair may require its own multi-potential guard. To the extentthat two conductors are at essentially the same potential and they arebalanced within the inner guard shield with respect to leakage and noisevoltages, they may be simultaneously protected within the samemulti-potential guard. This should be regarded as a special case whichmay not apply in a given application. The conventional single potentialguard of FIG. 1 also shares this limitation.

Third, the multi-potential guard disclosed herein is a guard and not ashield in the sense that a coax cable carrying video signals has ashield within which electromagnetic fields propagate and by whichoutside signals are attenuated. An additional shield, grounded at eachend, is required around the multi-potential guard to provide this shieldfunction. The conventional single potential guard of FIG. 1 also sharesthis limitation.

Fourth, the practical capabilities of the guard-drive operationalamplifier can limit effectiveness of the multi-potential guardingtechnique. Real amplifiers lack infinite gain and bandwidth and haveinput offset voltages and currents which influence overall guardquality. The conventional single potential guard of FIG. 1 also sharesthis limitation.

Theoretically, there is no limit to the dimensions of a guarded regionor to the number of guard groups in a system. A multi-potential guardcan enclose a surface area or a volume and apply to a portion of anintegrated circuit, a current carrying or potential sensing conductor, atest probe assembly in a wafer test system, an electronic instrument,even to a building or a signal transmission cable.

The most significant consideration to be observed in applying themulti-potential guarding technique is the need for the inner guardshield to act in effect as a single equipotential node. Therefore,wavelength and propagation delay effects must be considered when highfrequencies are involved. Also, the guard shields should be a few ordersof magnitude more conductive than the finite impedance of the insulationin the system.

Another practical consideration is the need for the inner guard shieldto receive no net charge from the region between the inner and outerguard shields. But, any outer guard can itself be treated as an innerguard shield by the application of an additional driven guard shield toprevent inter-guard charge transfer in unusual situations.

Practical operational amplifiers require a path for input bias currentsto flow. A sufficient bias current path may exist through poorinsulation for operational amplifiers serving in multi-potential guardapplications. But care should be taken to assure that an appropriatebias current path exists where insulation for direct currents isacceptable (e.g. when the purpose of the multi-potential guard is tominimize capacitive leakage or to deal with self-generating parasiticvoltages between a driven guard shield and the conductor it guards).

EXPERIMENTAL VERIFICATION

Two lumped-parameter models were constructed to demonstrateexperimentally the effectiveness of the multi-potential guardingtechnique of this invention. In the charge channel model, severalleakage components were included to permit significant variations in thecurrent distribution among the inner guard components. In the gradientguard model, a source of potential difference between a conductor andits guard was varied with leakage impedance included to limit theresulting current.

In each experiment, the circuit model was varied by attaching passiveand active test shunts to elements of the network. Both legal andillegal conditions were simulated by the active test shunt. Theautomatic change of the inner guard node potential to result inessentially no current through impedance Z1 and the constant voltageobserved across the load demonstrate the effectiveness of themulti-potential guarding technique in both models for legal conditions.

The first experiment was designed using lumped-parameter impedances todemonstrate the charge channel application of the multi-potentialguarding technique. The test circuit schematic diagram, includingcomponent values, is shown in FIG. 4 of the drawings. The results of thefirst experiment are presented in Table 1. The circuit simulates a highresistance (2 KΩ) conductor 40 having extremely poor insulation (100KΩ). The conductor and insulation impedances can be drastically loweredby paralleling them with a passive test shunt resistance (1 KΩ). Aconstant current is applied to develop a voltage gradient as it passesthrough the conductor 40. Without activating the guard system, only partof the input current flows through the conductor to a load resistor 42.The remainder of the current flows to ground through resistors 43-45which are chosen to simulate poor insulation. When the guard system isactivated, the same current level provided by the source should alsoflow through the load resistance 42. The guard drive amplifier 46 shouldoperate to make R₁, the simulated impedance between the inner and outerguard shields 48 and 50, appear to be an open circuit.

Metal-film components with 1% tolerance and 100 ppm/° C. stability wereused to construct and vary the circuit of FIG. 4. Components withgreater stability and accuracy were not deemed necessary because of thelarge changes to be made in the circuit and particularly because thereliability of the technique is indicated by the magnitude of anychanges that occur in the voltage drop, V_(out), across load resistor42.

The excitation current was adjusted with resistors 43-45 disconnectedand without leakage to develop a 1.000 V drop across the 1 KΩ loadresistor 42 using a digital voltmeter with an input resistance of 10 MΩ.Ideally, when resistors 43-45 are connected, the voltage drop acrossresistor 42 will remain at 1.000 V when the gradient-guard drive fromthe output of the operational amplifier 46 is connected to the outerguard node 52 and the various other resistances in the circuit areindividually shunted. The outer guard node 52 should experience widevariations in potential as the operational amplifier 46 acts to maintainthe charge channel.

When the output of operational amplifier 46 is disconnected from theouter guard node 52, the voltage drop across load resistor 42 isexpected to be less than 1 V and to experience significant variations asthe several circuit elements are individually shunted with 1 KΩ. Anactive test shunt was also used with the circuit of FIG. 4, consistingof a 100 KΩ resistor in series with a single AA alkaline cell. Similarresults were obtained, except for the case of shunting from junctions 57to 58. This illegal test violates the condition that the inner guardshield 48 receives no charge from outside the region it serves to guard.

The second experiment was designed using lumped impedances and a voltagesource to demonstrate a rejection by the multi-potential guardingtechnique of conductor-to-guard impedance and voltage variations. Thetest circuit schematic diagram, including component values, is shown inFIG. 5 of the drawings. The electronic components, test equipment andprocedure used were the same those employed in the first experimentdescribed above when referring to FIG. 4.

The input voltage, V_(in), is adjusted to result in an indication,V_(out), of 1.000 V with the guard system activated. Again, both passiveand active shunts were used to provide circuit variations. V_(out) isexpected to remain at 1.000 V when the gradient-guard drive from theoutput of the operational amplifier 46 is connected to the outer guardnode 60 and various resistances in the circuit are legally shunted. Theouter guard node 60 should experience wide variations in potential asthe operational amplifier acts to maintain the charge channel.

Tables 1 and 2 listed below present the data validity checks and theexperimental results. The various test conditions during the experimentare listed along with the corresponding output voltage, V_(out), and thevoltage at the output of the guard-drive amplifier 46, as applicable.

The results from the first experiment presented in Table 1 demonstratethat the charge-channel guard of FIG. 4 does indeed cause significantvariations in potential gradients and leakage impedances to have aninsignificant impact on the voltage drop across load resistor 42. Thevariations in the output voltage were below 100 ppm (the resolution ofthe voltmeter observing the output) while the guard node 52 assumedpotentials ranging from 1.0285 V to 2.9220 V. The constant voltageacross load resistor 42 in the first experiment in the presence of quitelarge variations in conductor and leakage impedance demonstrates that acharge channel was, in effect, created for the excitation current.Connecting the active shunt between junctions 54 and 55 violates therequirement that no charge may be arrive at the inner guard shield 48from outside the guarded region. As expected, the output voltage variesfrom ideal in this situation.

The results from the second experiment presented in Table 2 demonstratethat the gradient guard of FIG. 5 does indeed cause significantvariations in potential gradients and leakage impedances to have aninsignificant impact on the output voltage, V_(out). The constant outputvoltage, V_(out), in the second experiment in the presence of quitelarge variations in noise voltage and leakage impedance demonstratesthat a gradient guard was, in effect, created to transmit the inputvoltage. Again, connecting an active shunt between nodes 63 and 64violates the requirement that no charge may arrive at the inner guardshield 48 from outside the guarded region. As expected, the outputvoltage varies from ideal in this situation.

The second experiment also demonstrates how a real (non-ideal)operational amplifier 46 can cause difficulties. The amplifier in usehad an input offset voltage of about 100 microvolts. This appeared as aconstant, non-zero potential difference between the inner and outerguard shields 48 and 50. With average (e.g. 100 KΩ) insulation betweenthe inner and outer guards 48 and 50, the current (100 microvoltsdivided by 100 KΩ) injected in the inner guard node 61 wasinsignificant. However, with very poor insulation (1 KΩ), the currentbecomes significant as indicated by the 11 mV (about one percent) changein V_(out).

The experiments described above were designed to demonstrate the abilityof the multi-potential guarding technique and some of its limitations,not to model particular applications. The experiments used valuesrepresenting unusually large wire and small insulation-impedancemagnitudes along with a large noise voltage all undergoing hugevariations. Nevertheless, variations in the output voltage weretypically unobservable.

In summary, the classical driven guard technique of FIG. 1 applies onlyto electronic circuit regions that contain a single potential. Themulti-potential guarding technique of this invention uses a driven outerguard to effectively guard regions within an inner guard that containvarious potential gradients and noise sources. The gradient-guardconfiguration constrains charges within an inner guard shield and thecharge-channel configuration assures that all of the current entering aconductor at one end will exit the conductor at its other end even whenthe conductor and its guards are embedded in poor insulation resistanceto ground. The multi-potential guarding technique disclosed herein canalso significantly reduce the externally-observable effects of noisepotentials within the guarded region which may arise from tribo-electriceffects, radiation exposure, electromechanical inputs and other noisesources.

                                      TABLE 1                                     __________________________________________________________________________    Charge channel test results                                                                            Passive Shunt =                                                                         Active Shunt =                                                 No Guard                                                                           1 KΩ                                                                              1.53V + 100 KΩ                                           V.sub.out                                                                          Guard Node                                                                          V.sub.out                                                                         Guard Node                                                                          V.sub.out                            Test Condition      (Volts)                                                                            (Volts)                                                                             (Volts)                                                                           (Volts)                                                                             (Volts)                              __________________________________________________________________________    Excitation-off zero (noise check)                                                                 0.0000                                                                             0.0000                                                                              0.0000                                                                            --   --                                    Excitation on on, no leakage network:                                         Constant I directly to RL (1 KΩ load)                                                       1.0000                                                                             --    --  --   --                                    Constant I source check (additional 2 KΩ load)                                              1.0000                                                                             --    --  --   --                                    Excitation and guard drive on,                                                indicated network nodes shunted                                               No Shunt            0.9915                                                                             1.9915                                                                              1.0000                                                                            1.9915                                                                             1.0000                                54 to 55            0.9943                                                                             1.6627                                                                              1.0000                                                                            1.9916                                                                             1.0000                                55 to 56            0.9922                                                                             1.9915                                                                              1.0000                                                                            1.9575                                                                             1.0000                                54 to 57            0.9949                                                                             1.0285                                                                              1.0000                                                                            1.3533                                                                             1.0000                                55 to 57            0.9902                                                                             1.9907                                                                              1.0000                                                                            1.6104                                                                             1.0000                                56 to 57            0.9856                                                                             2.9220                                                                              1.0000                                                                            1.8577                                                                             1.0000                                57 to 58            0.9853                                                                             1.9881                                                                              1.0000                                                                            2.5255                                                                             0.9617                                58 to 59            0.9813                                                                             1.9915                                                                              1.0000                                                                            1.9915                                                                             1.0000                                __________________________________________________________________________

                                      TABLE 2                                     __________________________________________________________________________    Gradient guard test results                                                                                       Active Shunt =                                                No Guard                                                                           Passive Shunt = 1 KΩ                                                               1.53V + 100 KΩ                                          V.sub.out                                                                          Guard Node                                                                          V.sub.out                                                                          Guard Node                                                                          V.sub.out                           Test Condition      (Volts)                                                                            (Volts)                                                                             (Volts)                                                                            (Volts)                                                                             (Volts)                             __________________________________________________________________________    Excitation-off zero (guard amplifier removed)                                                     0.0000                                                                             0.0000                                                                              0.0000                                                                             --    --                                  Excitation on, no noise or leakage network:                                                       1.0000                                                    Voltage source directly to meter                                                                  1.0055                                                                             --    --   --    --                                  Voltage source through 100 KΩ to meter                                                      0.9955                                                                             --    --   --    --                                  Excitation and guard drive on,                                                indicated network nodes shunted                                               No shunt            0.7483                                                                             1.0143                                                                              1.0000                                                                             --    --                                  62 to 63            0.6669                                                                             1.0099                                                                              1.0000                                                                             1.7782                                                                              1.0000                              63 to 64            0.6668                                                                             0.9944                                                                              0.9890                                                                             4.0770                                                                              2.5170                              64 to 65            0.6668                                                                             1.0142                                                                              1.0000                                                                             1.0144                                                                              1.0000                              __________________________________________________________________________

What is claimed is:
 1. A method for guarding an electrical systemcontaining potential gradients and sources of electrical noise so as toprevent undesired electrical charge exchange to or from the system, saidmethod comprising the steps of:surrounding the system by an innerelectrically conductive guard having an electrical potential;surrounding said inner electrically conductive guard by an outerelectrically conductive guard; and driving said outer electricallyconductive guard to an electrical potential that is substantiallyidentical to the electrical potential of said inner electricallyconductive guard so that there is substantially no potential differencebetween said inner and outer electrically conductive guards, whereby toeffectively produce a high insulation impedance around the system. 2.The method for guarding recited in claim 1, wherein the electricalpotential of said inner electrically conductive guard relative to theelectrical potential to which said outer electrically guard is drivencauses essentially no net current flow to said inner electricallyconductive guard from the electrical system located therewithin.
 3. Themethod for guarding recited in claim 1, including the additional step ofdriving said outer electrically conductive guard by means of anoperational amplifier having a high input impedance and a low outputimpedance.
 4. The method for guarding recited in claim 3, wherein saidoperational amplifier has output means and inverting and non-invertinginput means, said inner electrically conductive guard being connected tothe non-inverting input means of said operational amplifier, while theoutput means and the inverting input means of said operational amplifierare connected to said outer electrically conductive guard.
 5. The methodfor guarding recited in claim 1, wherein a first electrical impedanceexists between said inner and outer electrically conductive guards, anda second electrical impedance exists between said outer electricallyconductive guard and electrical ground.
 6. The method for guardingrecited in claim 1, including the additional step of arranging the innerelectrically conductive guard to lie inside and spaced from the outerelectrically conductive guard so that a region of high impedance isestablished in the space between said inner and outer electricallyconductive guards.
 7. The method for guarding recited in claim 1,wherein the electrical system surrounded by said inner electricallyconductive guard includes a current carrying wire that is connected toan electrical load impedance that senses a physical condition such thatthe currents entering and leaving said conductor are identical.
 8. Themethod for guarding recited in claim 7, wherein said electrical loadimpedance is adapted to measure temperatures.
 9. A method for guardingan electrical conductor that carries information and travels through aregion containing potential gradients and sources of electrical noisesuch that substantially all of the information carried by the electricalconductor into the region is also carried by the electrical conductorout of the region, said method comprising the steps of:surrounding theelectrical conductor by an inner electrically conductive guard having anelectrical potential; surrounding said inner electrically conductiveguard by an outer electrically conductive guard; and driving said outerelectrically conductive guard to an electrical potential that issubstantially identical to the electrical potential of said innerelectrically conductive guard so that there is substantially nopotential difference between said inner and outer electricallyconductive guards to effectively produce a high insulation impedancearound the electrical conductor, whereby parasitic electrical potentialsthat occur within the inner electrically conductive guard are unable tocause an electrical charge to flow outside said inner electricallyconductive guard to alter the information carried by said electricalconductor.
 10. The method for guarding recited in claim 9, including theadditional steps of driving said outer electrically conductive guard bymeans of an operational amplifier having a high input impedance, a lowoutput impedance, output means, and inverting and non-inverting inputmeans;connecting said inner electrically conductive guard to thenon-inverting input means of said operational amplifier; and connectingsaid outer electrically conductive guard to the inverting input meansand the output means of said operational amplifier.
 11. The method forguarding recited in claim 9, wherein the information carried by theelectrical conductor through the region containing potential gradientsand sources of electrical noise is a current having a magnitude, one endof said electrical conductor connected to an electrical load impedancethat senses a physical condition, and the opposite end of saidelectrical conductor connected to a source of constant current, theeffective high insulation impedance around the electrical conductorcausing the magnitude of the current being carried through said regionby said conductor to remain substantially constant.
 12. The method forguarding recited in claim 11, where in said electrical load impedance isadapted to measure temperatures.